Anatomy of the Human Eye - How the Human Eye Works
By Tenay Brown
The human eye is commonly referred to as the human body's most complex organ. It contains many different parts and depends on all parts to work efficiently in order to have the best vision possible. Of all five senses, vision is the most valued and the quality of this vision is directly related to the way the eye's parts work with each other.
Basic Eye Anatomy
The eye consists of 11 basic parts, all of which have a vital role in the vision process. The basic anatomy of the eye includes the iris, cornea, pupil, lens, sclera, ciliary body, choroid, optic nerve, macula, retina and vitreous. Light enters through the cornea, which is the region of the sclera that is transparent. It provides the eye with the power to focus and is also the part of the eye where contact lenses are placed. The color of your eye, or the colored iris, is located behind the cornea and is visible only because the cornea is transparent.
In the center of the iris is the pupil, which is the dark and round opening in the center. It has control over how much light enters into the eye by dilating the size of the pupil in conditions of dim light and constricting in conditions of bright light. The lens also provides some focusing power within the eye and is located directly behind the pupil. The shape of the lens changes to bring objects into focus-such as when you are reading-and muscles contract that are located in the ciliary body. After a while, the muscles lose the ability to change shape and the lens becomes rigid which leads to a need for glasses.
Light images are converted into electric pulses by photosynthesis cells that make up the retina. These electric pulses then travel to the brain through the optic nerve. Fine detailing is accomplished by the macula, which is a specialized area that is located in the center of the retina. The clarity of vision is determined by a combination of factors: the length of the eye and the focusing power of both the lens and the cornea. Vision is clear if these three factors match perfectly together; when they do not, this creates vision problems.
Sclera
The sclera, also known as the white part of the eye, is the protector of more sensitive parts of the inner eye, such as the choroid and the retina. Most of the depth of the sclera is 0.03-inch thick, but it is no more than 0.01-inch thick where the straight eye muscles connect. There are six muscles in each eye that control the movements, but four are referred to as the straight eye muscles. The sclera is nourished with nutrients and oxygen through blood vessels located in the episclera, which is located on top of the sclera.
Choroid
The choroid provides the retina with a nourishment of nutrients and oxygen through a network of blood vessels. It is located within the sclera and is responsible for providing the anterior part of the optic nerve as well as the macula with the blood supply they depend on. When there is an abnormal growth of blood vessels, an eye condition called macular degeneration occurs, beginning in the choroid and going to the retina through the Bruchs Membrane. Because of a weaker structure, this causes edemas due to the bleeding or rupturing of the weakened vessels.
Retina
The retina is in the back of the eyeball and is the light-sensitive tissue that contains two types of light receptors: cones and rods. The cones generally absorb the stronger light and are color-sensitive, located in the retina's center. The rods absorb black and white in soft light and are next to the fovea. Retinal detachment, or amotio retinae, is a serious eye condition that produces symptoms such as light flashes or vision loss that can lead to permanent vision loss if left untreated.
Iris
The iris has control over how much light enters into the eye; the muscles of the iris sphincter with contract with strong light and will dilate with dim light. The diameter of the iris expands when focusing on an object far away and narrows when focusing on near objects-this is called the accommodation reflex. The eye color is a direct relation to the type and amount of pigments in a person's iris. Green is the least common color while brown is the most common.
Cornea
The cornea covers the anterior chamber, the pupil and the iris. It is transparent and contains two-thirds or 40 of the 60 diopters in the eye. It is supplied with nutrients and oxygen through tear fluid instead of blood vessels, which explains why it is so clear. Blood vessels can appear in the cornea though if there is an oxygen deficiency which can happen from overusing contact lenses. This can cause partial vision loss if the vessels grow too close to the center. Surgical procedures can be performed to correct eye conditions of the cornea such as astigmatism, hyperopia and myopia.
Pupil
The pupil is the hole in the center of the eye that absorbs major parts of light, which is the reason it is black. When they appear red in pictures, it is due to the reflection of the retina.
Vitreous Body
This part of the eye is located behind the lens and fills the eye with a clear and gel-like substance. The vitreous body shrinks as you gets older and can detach from the retina as the body decreases in size. Vitreous body detachment is generally not a dangerous condition, but can become dangerous if it drags along the retina.
Lens
One-third of the eye's diopters obtain their power in the lens while the remainder is in the cornea. The lens is the part of the eye that allows you to change focus depending on the distance of an object whether it is across the room or directly in front of your face. It is attached to zonula threads which are attached to the ciliary body. When focusing on a near object, the ciliary body contracts, which allows the zonula threads to loosen and the lens to increase its acuity by thickening in diameter while contracting. Cataracts is an eye condition that is age-related and results in blurry vision as the most common symptom. The treatment for cataracts is generally surgically removing and replacing the lens.
Ciliary Body
The ciliary body contains the ciliary muscle, which is responsible for controlling the vision accommodation reflex. The body is also an attachment point for the zonula threads and produces aqueous fluid.
Optic Nerve
The optic nerve is responsible for sending signals to different areas of the brain from the eye in order to create images. It consists of about one million nerve threads and the nerves from both eyes are connected behind them so that the left field of vision is sent to the right hemisphere of the brain and vice verse.
Macula
The macula consists of visual cells that are closely packed resulting in high visual acuity, or high image resolution. These cells are not as closely packed around the macula as they are inside of the actual macula, but they control how well the peripheral vision works and is good enough for it to work properly. Macular degeneration is a common eye disease that occurs among the elderly and is separated into two categories: wet and dry. Wet is more serious than dry, but is also treatable with FDA-approved drugs while dry is not treatable and is the most common.
Basic Eye Anatomy
The eye consists of 11 basic parts, all of which have a vital role in the vision process. The basic anatomy of the eye includes the iris, cornea, pupil, lens, sclera, ciliary body, choroid, optic nerve, macula, retina and vitreous. Light enters through the cornea, which is the region of the sclera that is transparent. It provides the eye with the power to focus and is also the part of the eye where contact lenses are placed. The color of your eye, or the colored iris, is located behind the cornea and is visible only because the cornea is transparent.
In the center of the iris is the pupil, which is the dark and round opening in the center. It has control over how much light enters into the eye by dilating the size of the pupil in conditions of dim light and constricting in conditions of bright light. The lens also provides some focusing power within the eye and is located directly behind the pupil. The shape of the lens changes to bring objects into focus-such as when you are reading-and muscles contract that are located in the ciliary body. After a while, the muscles lose the ability to change shape and the lens becomes rigid which leads to a need for glasses.
Light images are converted into electric pulses by photosynthesis cells that make up the retina. These electric pulses then travel to the brain through the optic nerve. Fine detailing is accomplished by the macula, which is a specialized area that is located in the center of the retina. The clarity of vision is determined by a combination of factors: the length of the eye and the focusing power of both the lens and the cornea. Vision is clear if these three factors match perfectly together; when they do not, this creates vision problems.
Sclera
The sclera, also known as the white part of the eye, is the protector of more sensitive parts of the inner eye, such as the choroid and the retina. Most of the depth of the sclera is 0.03-inch thick, but it is no more than 0.01-inch thick where the straight eye muscles connect. There are six muscles in each eye that control the movements, but four are referred to as the straight eye muscles. The sclera is nourished with nutrients and oxygen through blood vessels located in the episclera, which is located on top of the sclera.
Choroid
The choroid provides the retina with a nourishment of nutrients and oxygen through a network of blood vessels. It is located within the sclera and is responsible for providing the anterior part of the optic nerve as well as the macula with the blood supply they depend on. When there is an abnormal growth of blood vessels, an eye condition called macular degeneration occurs, beginning in the choroid and going to the retina through the Bruchs Membrane. Because of a weaker structure, this causes edemas due to the bleeding or rupturing of the weakened vessels.
Retina
The retina is in the back of the eyeball and is the light-sensitive tissue that contains two types of light receptors: cones and rods. The cones generally absorb the stronger light and are color-sensitive, located in the retina's center. The rods absorb black and white in soft light and are next to the fovea. Retinal detachment, or amotio retinae, is a serious eye condition that produces symptoms such as light flashes or vision loss that can lead to permanent vision loss if left untreated.
Iris
The iris has control over how much light enters into the eye; the muscles of the iris sphincter with contract with strong light and will dilate with dim light. The diameter of the iris expands when focusing on an object far away and narrows when focusing on near objects-this is called the accommodation reflex. The eye color is a direct relation to the type and amount of pigments in a person's iris. Green is the least common color while brown is the most common.
Cornea
The cornea covers the anterior chamber, the pupil and the iris. It is transparent and contains two-thirds or 40 of the 60 diopters in the eye. It is supplied with nutrients and oxygen through tear fluid instead of blood vessels, which explains why it is so clear. Blood vessels can appear in the cornea though if there is an oxygen deficiency which can happen from overusing contact lenses. This can cause partial vision loss if the vessels grow too close to the center. Surgical procedures can be performed to correct eye conditions of the cornea such as astigmatism, hyperopia and myopia.
Pupil
The pupil is the hole in the center of the eye that absorbs major parts of light, which is the reason it is black. When they appear red in pictures, it is due to the reflection of the retina.
Vitreous Body
This part of the eye is located behind the lens and fills the eye with a clear and gel-like substance. The vitreous body shrinks as you gets older and can detach from the retina as the body decreases in size. Vitreous body detachment is generally not a dangerous condition, but can become dangerous if it drags along the retina.
Lens
One-third of the eye's diopters obtain their power in the lens while the remainder is in the cornea. The lens is the part of the eye that allows you to change focus depending on the distance of an object whether it is across the room or directly in front of your face. It is attached to zonula threads which are attached to the ciliary body. When focusing on a near object, the ciliary body contracts, which allows the zonula threads to loosen and the lens to increase its acuity by thickening in diameter while contracting. Cataracts is an eye condition that is age-related and results in blurry vision as the most common symptom. The treatment for cataracts is generally surgically removing and replacing the lens.
Ciliary Body
The ciliary body contains the ciliary muscle, which is responsible for controlling the vision accommodation reflex. The body is also an attachment point for the zonula threads and produces aqueous fluid.
Optic Nerve
The optic nerve is responsible for sending signals to different areas of the brain from the eye in order to create images. It consists of about one million nerve threads and the nerves from both eyes are connected behind them so that the left field of vision is sent to the right hemisphere of the brain and vice verse.
Macula
The macula consists of visual cells that are closely packed resulting in high visual acuity, or high image resolution. These cells are not as closely packed around the macula as they are inside of the actual macula, but they control how well the peripheral vision works and is good enough for it to work properly. Macular degeneration is a common eye disease that occurs among the elderly and is separated into two categories: wet and dry. Wet is more serious than dry, but is also treatable with FDA-approved drugs while dry is not treatable and is the most common.
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